1. Field of the Invention
The present invention relates to an electromagnetic mark device for a magnetic encoder, and, particularly, to a device, which can generate a magnetic field by way of the current to make magnetic marks much easily, such that it is possible to reduce the pitch between any two neighboring magnetic marks greatly so as to enhance the accuracy of the magnetic encoder in addition to the magnetic field being set up reliably with stable magnetic flux.
2. Description of Related Art
Generally, a technical application of an encoder is to measure the relative position, the velocity and the angular velocity of an object such that the object can be located more accurately. The principle thereof is to fix a series of magnetic marks equidistantly on a base, and a magnetic resistance element is utilized to pass through the magnetic lines of force on each magnetic mark so as to sense a change of signal for determining a relative position of a moving object. A measure apparatus thereof is usually called an encoder, wherein, the apparatus used for measuring a linear position of the object is called a linear encoder and for measuring a rotational angle of the object is called an angular encoder.
A conventional encoder as shown in FIG. 1 comprises a stationary ruler body 101, two opposite guard layers 102, 103 disposed at the upper and the lower ends of the ruler body 101, and a sensor 104. A plurality of magnetic marks N, S, N, S, . . . with a constant pitch arranged on the ruler body 101 in a way of adjacent magnetic poles of the magnetic marks being disposed alternately. The sensor 104 is fixedly attached to a moving mechanism and provided with two magnetic resistance sensing elements 104A, 104B. When the moving mechanism carries the sensor 104 to move along the ruler body with a relative constant velocity, the sensing elements 104A, 104B have different resistances during passing through different magnetic fields on the ruler body 101. Signals on the sensing elements 104A, 104B as shown in FIG. 2 includes a signal approximate sine wave and a signal approximate cosine wave respectively. The signals can be treated and figured out the number of pulse waves such that it is possible to find out the displacement of the moving mechanism relative to the stationary mechanism. Wherein, in case of the moving mechanism displacing in a direction designated as 105A, the sensing element 104A phase-leads the sensing element 104B, and, in case of the moving mechanism displacing in a direction designated as the sensing element 104A phase-lags the sensing element 104B. Hence, it is possible to determine if the moving mechanism is turning forward or turning inversely based on the state of phase thereof.
It can be learned from the preceding analysis that the encoder has the resolution thereof to be determined by way of the capability of picking up the signal from the sensor and the accuracy of the magnetic marks on the ruler body. But, problems caused by both of the magnetization technique and the magnetic material result in that it is hard to shorten the pitch between magnetic poles. The prior art mostly emphasizes on the improvement of the capability of picking up the signal of sensor such as the selection of magnetic resistance material and the arrangement of signals in the sensor, or on the improvement of the manufacturing process to the sensibility of flux, the anti-noise of flux and the characteristics of temperature change. However, the prior art provides less improvement on the ruler body making and the mode for producing the magnetic marks.
For producing the magnetic marks on the ruler body, the magnetic material in the prior art is mostly utilized to coat the ruler body and then the magnetic marks are produced by way of magnetization. Theoretically, the smaller the graduations of the magnetic marks are, the higher the resolution will be, but, smaller graduations of magnetic marks may confront many problems. Firstly, the magnetization work is required to up-grade for meeting the needed accuracy such that it becomes more difficult to produce the magnetic marks and the production cost for the magnetic marks increases considerably. Next, a smaller pitch may increase the error percentage of the magnetic marks under the condition of same error such that it requires a higher working accuracy for maintaining the same error percent. Hence, the density of coating on the magnetic material has to be increased relatively. Further, it is hypothesized that the ruler body is made of homogeneous material at the time of being designed and the hypothesis is proper in case of the pitch being much greater than the grain size of the magnetic material. While the pitch is shortened to be close to the grain size of the magnetic material, it may affect the intensity of magnetic field to generate noise if the magnetic material is not so homogeneous as the original hypothesis. In order to avoid the noise generated from the material, a smaller grain size and a more strict homogeneity have to be reached synchronously with the reduced pitch such that the material is required to provide a better homogeneity as soon as the graduations of the magnetic marks become smaller. In practice, the magnetic marks of the ruler body usually are magnetized on a magnetic rubber and the pitch is restricted in a range of 5 mm to 10 mm due to being affected by the grain size and the homogeneity of the material. If the pitch is required to reduce further, a special magnetic material has to be plated on a hard base plate as a medium for magnetic marks. However, the special magnetic material mostly is made of alloy so that, on the one hand, it is not easy to be coated on the magnetic material and, on the other hand, it is easy to be scratched. Moreover, the surface of the special material may crack because of the factor of temperature. Unfortunately, the signal of the encoder may arise noise regardless the crack or the scratch. Secondly, in addition to the limitation concerning the characteristics of the material, the smaller the magnetic graduation is, the stricter the control with regard to the density and the accuracy of the magnetization is required. Hence, the accuracy of the magnetic marks on the ruler body is hard to be enhanced due to the restricted technique of magnetization.
Besides, the permanent magnetic material is normally utilized to produce the magnetic marks on the ruler body but in case of being adsorbed with magnetic dust, being not uniform at the interior thereof, being scratched at the outer surface thereof, or being interfered by the foreign magnetic field, the permanent magnetic material may generate the noise. Further, the smaller magnetic graduation needs a small corresponding signal but the noise still remains unchanged. Moreover, the smaller magnetic graduation may results in a smaller signal/noise ratio and the incorrect signal will be generated undesirably. Further, the permanent magnetic material can be demagnetized by the interference of a more powerful external magnetic field but the smaller magnetic graduation may result in a smaller magnetic flux and a smaller magnetic force needed for the demagnetization. Hence, Once strong lines of magnetic force exist in the work environment, the magnetic marks has an increased possibility of being demagnetized relatively so that it has a certain limit to reduce the graduation. In practice, a lot of uncertain lines of magnetic force may distribute over around an environment in which motors are in use in addition to many magnetic material being able to generate dust such that extraordinary magnetic marks may be influenced by the apertures in the magnetic material, scratches on the ruler body, the adsorption of the magnetic dust and the interference of the magnetic field to produce incorrect signal. Furthermore, the reduced magnetic graduation may result in the permanent magnetic material being demagnetized or magnetized improperly and it may cause the permanent damage if the ruler body is carelessly exposed to the strong magnetic field during setting up the encoder. Therefore, the preceding internal and external problems are always there if the permanent magnetic field is utilized as marks and the manufacturers are incapable of solving the problems effectively.
Accordingly, an object of the present invention is to provide an electromagnetic mark device for a magnetism type linear and curvilinear encoders, which can generate a magnetic field by way of electric current, so as to shorten the magnetic gap between the magnetic marks greatly so that the resolution thereof can be enhanced to promote accuracy thereof during performing a detection job for the position of an object.
Another object of the present invention is to provide an electromagnetic mark device for a magnetism type linear and curvilinear encoder, in which the magnetic field intensity is independent of the homogeneity of material thereof and not demagnetized by the external magnetic field, so as to assure the position of an object being detected stably.
A further object of the present invention is to provide an electromagnetic mark device for a magnetism type linear and curvilinear encoder, which can be fabricated by way of photo etching, so as to lower down the production cost of the precision magnetic ruler with high resolution.
According to the present invention, the concept of electromagnetism with regard to the characteristics of a plane perpendicular to the electric current being capable of forming an annular magnetic field is applied so that a base plate of a ruler body for a framework of magnetic marks is arranged with a lead wire to divide the ruler body with a fixed pitch. As soon as the current passes through the lead wire, adjacent lead wire sections thereof can produce a magnetic field with continuous north poles (N) and south poles (S) being distributed alternately such that alternate magnetic marks Nxc2x7Sxc2x7Nxc2x7S . . . are formed on a flat surface of the ruler body instead of the magnetic ruler made of permanent magnetic material to offer the same effect as the permanent ruler body does. For instance, the lead wires can be formed as a continuous S-shaped wave to divide the ruler body with a fixed pitch and the manufacturing process in practice is to etch the base plate with the technique of photo etching such that 0.004xe2x80x3 width lead wire and 0.006xe2x80x3 (0.15 mm) pitch of S-shaped wave can be formed. Hence, the present invention can provide magnetic marks on the ruler base plate with a pitch being reduced to 0.15 mm and it is not possible for the prior art to reach so that the resolution thereof can be enhanced greatly and the detection for the position of an object can be performed more accurately. In addition, the magnitude of the magnetic force varies in accordance with the current so that it secure the magnetic field remaining unchanged as long as the current is kept to pass through the lead wires.